This invention relates to the degassing of a pulp slurry following oxygen bleaching. Bleaching is employed to delignify and whiten brown stock from a wood digestion process.
In recent years oxygen has often been used as the bleaching agent, in place of the chlorine that was formerly the principal chemical used. The use of oxygen reduces the effluent from the bleaching step. Bleaching processes employing oxygen are disclosed in U.S. Pat. Nos. 3,814,664, 3,832,276, 3,963,561, 3,964,962, and 4,022,654, all of which are hereby incorporated by reference. None of these, however, mentions the problems that can occur in the subsequent processing of an aqueous wood fiber slurry that has been oxygen-bleached. Degassing of residual oxygen could occur at an inopportune moment in such subsequent processing and cause the wood fibers to rise to the surface of the slurry, thereby forming a mat of such fibers, which hinders further processing.
Methods of removing gases from pulp slurries are suggested in U.S. Pat. Nos. 3,432,036 and 3,807,142. In the former, a number of hydrocyclones and a large vacuum tank are needed, which require a substantial capital outlay and a large amount of energy to maintain the vacuum. In the latter patent, a cyclone-type separator is used, but in actual operation, this type of separator has been found ineffective in removing residual oxygen.